In a known technique, a sheet on which an image is formed with what is called a decolorable colorant is heated to remove a color from the image on the sheet, thereby making the sheet reusable.
A sheet targeted for decoloring may change in its print density, thickness and others. A color can be removed satisfactorily from a thin sheet of a low print density without applying heat of a high temperature to the sheet.
Therefore, as a result of application of the same amount of heat in decoloring to sheets of various types containing various printed contents, some of the sheets may be heated excessively, causing an issue in terms of energy saving.
In a known structure, it is determined to which extent a color is removed after decoloring. The decoloring is performed again according to the extent. In this structure, however, the decoloring should be performed twice on the same sheet. Further, heat of an amount greater than necessary may be applied in the first decoloring.